Alloy steel



Reissued July 14, i953- UNITED ST-ATE ALLOY STEEL William Charles Clarke, Jr., Dundalk, Md., ss-

signor to Armco Steel, Corporation, a corporation of Ohio No Drawing. Original No. 2,528,637, dated November 7, 1950, Serial No. 768,718, August 14, 1947. Application for reissue February'l, 1951,

Serial Nil- 20931) Claims. (Cl. 148-31) Matter enclosed in heavy brackets appears in the original patent but forms no part oi this matter 1 reissue specification;

' 1705345, filed October 23, 1946, now Patent No.

2,523,497, and the invention relates to high temperaturestainless steel products and articles.

Among the objects of my invention is the provision of strong, tough and durable high tempera ture stainless steel articles which are resistant to stress-rupture and creep under load at the high temperatures encountered during use, and which are capable of resisting the development of heat scale and attack by hot corrosive matter.

' .Qther objects in part will be obvious and in part pointed out hereinafter.

The invention accordingly consists in the combination of elements, features of composition, and

in the several steps and the relation of each of thesame to one or more of the others as described herein, and the scope of the application of which is indicated in the following claims. As conducive to a clearer understanding of certain features of my invention, it may be noted at this point that stainless steel byt'general den-- nition contains about 10% to 35% chromium,

with-or without nickel and with or without additions of manganese, silicon, cobalt, copper, molybdenum, tungsten, vanadium, columbium, titanium, aluminum, zirconium and the like, for special purposes, and the remainder substantially all iron. ,These steels include anywhere from 'about'0.0l% up to about 0.25% carbon, and more of this element where the steels are of the high :carbon varieties. v

- There are ever increasing demands for high -temperature articles as for achieving functions at temperatures ranging from about 1000 F. to 1700" F. orhigher. "These demands sometimes associate the use of the articles with hot corroslve gases, liquids or solidswhich add to the sever- 1 lty ot thecondltlons to be withstood.

A' number of the articles now available in the prior art, for use at. high temperatures, therefore, sufl'er severe iniury' at the surface, this by active attack by corrosive chemicals. An example of this is had where the articles are in the form of parts for high temperature chemical equipment.

' sisting mechanical stress. I

less steels tend to creep and rupture under load printed in italics indicates the additions made by reissue.

2 Y sistance and their mechanical properties at low temperatures. In this connection, it can be said that articles made of martensitic or-l'erritlc stainless steels, in general have greater physical strength at low temperature than do those of the austenitlc grade. It might be expected that these properties would be retained at high temperatures. It nevertheless is a fact that articles made or the austenitic chromium-nickel stainless steels have a more favorable lattice structure for 00- hesion under high mechanical stress at temperatures above about 100 F. than do the martensiticor'ierritic stainless steels. Still, it is well recognized that certain austenitic steels are unsatisfactory for meeting exacting requirements of articles in high temperature use, particularly where these articles are to be relied upon .for re- The austenitic'stainwhile hot. Apart from these difliculties, however,

high temperature articles of the steels ofler a wide variety of beneficial properties, among these being corrosion resistance and a favorable, lattice structure for strength at high temperatures."

An outstanding object of my invention accordingly-ls the provision of high temperature aus- .tenitic stainless steel articles, which are .strong and capable of resisting the damaging streets of corrosion and heat and of withstanding creep and rupture while subjected to mechanical stress at elevated temperatures.

Referring now more particularly to the practice oi my'invention, I provide high temperature chromium-nickel stainless steel articles in which the steel importantly contains critical amounts of phosphorus and is substantially wholly austenitic, giving the surprisin properties of resist- In many instances, too, the intense heat leads to the development of large amounts of oxide or heat scale on the article surfaces. Also, certain 01' the artlclssare susceptible to creep or rupture under load a'wlth detriment to the functions served. I I

certain occasions stainless steel articles in theprior art have been resorted to for high tem- 'perature dlltl' on thebasis uf th'eir corrosion ance to creep and stress rupture. In addition to the chromium, nickel and phosphorus constit uents, sometimesthe steel contains supplemental amounts of one or more such elements as man"- ganese, cobalt, copper, molybdenum, tungsten, vanadium, columblum, titanium, aluminum, and the like, the remainder being substantially all iron. Ferrite, if present stall in the steel, is'only in traces preferably-not exceeding about 1.5% to 2% by volume. This virtual exclusion of ferrite, and the inclusion of critical amounts of the element phosphorus in the austenitic steel, I "find enables the use oi. my articles at temperatures even so high as 1200" F. to 1700 F. or more, and for resisting creep and stress rupture.

The austenitic chromium-nickel stainless steel. high temperature articles which I provide,

preferably contains in the steel compositionmbout 0.01% to 025% carbon, about 16% to 26% chro-.

mium, from 6% to 30% nickel, phosphorus ranging from 0.08% to 0.12%, and theremainder substantlally all iron. While the more limited phosphorus range just noted is preferred, the useful range at times extends from about 0.05% to 0.12% and even up to approximately 0.5%. On the low side of the more general range, however, the phosphorus content is nearly too low to lend appreciable improvement to the high temperature mechanical properties of the articles. When the phosphorus amounts to much beyond 0.12%, brittleness of the metal tends to become observable, but no complete loss of the beneficial effects of phosphorus on the high temperature stress resisting properties, is suifered.

Any additional elements which do not materially inhibit the austenitic structure or alter the beneficial eil'eots of phosphorus on the high temperature properties also many be present in the steel, if desired, as for serving special purposes. Such elements as manganese and silicon, while usually preferred in the steel articles in small or'incidental quantities, sometimes are ineluded in amounts up to 2% or appreciably more, as where the particular purpose of the articles so demands. There are'occasions where I partially replace or merely supplement the amounts of chtomium'with an element such .as molybdenum in the steel, and so produce the articles. The same holds true for nickel in the steel, as where for example'manganese or cobalt either or both afford a partial replacement or merely supplement given amounts of the element. At times, the carbon of the steel articles may extend beyond.0.25%,but this usually is not preferred since higher carbon. values often tend to inhibit working and fabricating the metal.

Examplesof certain more specific and preferred compositions of the high temperature articles which I provide are noted below in Table I. Itwill be'appreciated, with particular attention to the high-phosphorus steels employed, that the articles are producedwithout appreciable extra costof materials as compared h the cost of standard low-"phosphorus'grades of stainless steel.

' TABLE I. HIGH TEMPERATURE STAINLESS STEEL ARTICLE COMPOSITIONS Article Percent CR NI P I Other Elements 16-18 0.05415 17-19 8-10 0.05-0.5 17-10 8-10 0.0505 or Be, 0.07% Min;

M0 or Zr, 0.6% Max. 18-1] 8-10 0.05-0.15 19-21 -12 0. 06-05 22-24 12-15 0.D5-O.5 24-26 H42 0.0505 Si 1.50Max 16-18 10-14 0.05-0.5 M01.75-2.50 17-19 8-11 0.05-0.15 Min Tl-lOXC 17-19 9-12 0.05415 Mm Cb==8XC Norm-Articles A-J', inclusive, contain Mn 2% man, 81 1% max., 8 0.04% max., and remainder substantlallyeall iron, unless other amounts of these elements appear in the tab 4 amples of the austenltic phosphorus-containing stainless steel articles which I provide are thermocouple parts and temperature control instruments. Still further examples are exhaust supports, manifolds, screws, bands, and the like valve fittings to handle hot corrosive media,

chimney and flue structural elements, wire cloth 1 for chemical screening purposes at high temperatures, wire baskets and racks for metallurgical processing, and numerous other articles needed for withstanding corrosion and stress while heated.

Sometimes I accept the beneflclal and satisfactory eil'ects of phosphorus in my high temperature stainless steel articles without any thought of further developing these eflects. As an alternative, however, I frequently find advantage in subjecting the phosphorus-containing steel to special heat treatment including a combination of annealing and aging treatments for further enhancing the resistance to creep and stress rupture over and above the results gained by the presence of phosphorus in the relatively untreated steel. In the annealing portion of this treatment, I heat the steel, as for example the article itself, to a sufficiently high temperature and for long enough time for forming a solid-solution including phosphorus and'other components of the steel. A preferred-temperature range for this purpose is that of about 2000 F. to 2250 F. at these temperatures, a holding time of about one-half hour is usually adequate for solubility.

Following the heating for solubility, I quench the steel, conveniently to about room temperature, this for. example while using air, oil or water'as the quenching medium.

After the annealing and quenching steps of. my special heat treatment, I re-heat the stainless steel, this time to an optimum temperature of about 1300 F., or advantageously within an approximate temperature range of 1200 F. to 1500 F., for precipitation treatment. Upon holding the metal at temperature for a sufllcient period of time, preferably for a period of at least .about five hours, precipitates accrue in the matrix. These precipitates. are finely divided and are critically dispersed in the metal lattice along the atomic slip planes. They are thought to include intermediate compounds. It is believed that the phosphorus in solution inhibits the diffusion of carbides. There is some evidence too that the phosphorus comes out of solution with chromium and as a chromium compound.

At the conclusion of the precipitation treatment, I quench the steel. The quenched metal has enhanced load carrying capacity and improved creep resistance in view of atomic slip interference exerted by the precipitates. These precipitates remain uncoalesced between the atomic slip planes and effective against creep and stress rupture of the steel for extremely long periods of time at high temperatures.

As illlustrative of the practice of my inflation, I provide high-phosphorus austenitic steel containing about 0.098% carbon, 15.90%, chromium, 13.80% nickel, 0.098% phosphorus, 2.99% molybdenum, 1.20% manganese, 0.51% silicon, 0.015% sulphur, and the remainder substantially all iron. This steel for example isin the form of an ingot which I work into small billets by forging at a suitable temperature. Then, by such operation as hot-rolling, I produce boiler rods of the steel, these illlustratively being in rough form. Should the rods require flanges near the. ends, I sometimes apply these using the same type of steel and by welding with an oxyacetylene torch or by electric are means, the welding rods for this purpose preferably being of the same analysis as the parent metal. The roughly formed articles conveniently are heated to annealing temperature and quenched. In the present instance, this is achieved by heating the steel 1362250 F., holding the metal at this temperature for about one-half hour, and thereafter quenching in water. For subjecting the metal to preciiitation treatment, I heat the annealed boiler ro s to a temperature of about 1200 F., and maintain this temperature for approximately five hours. At the end of this treat ment, I quench the rods in water. Machining of the rods to finished dimensions and thread ing the ends are undertaken where desired, either before or after the precipitation treatment. Upon being finished and put into use, the rods are resistant to heat and corrosion. They are capable of enduring at 1200 F., a load of 33,000 p. s. i. for 105 hours, a load of 30,800 p. s. i. for 446 hours, and a load of 30,000 p. s. i. for 1981 hours. These values extrapolated logarithmically reveal that, at the same temperature, the

rods can endure a load of 28,000 p. s. i. for 10,000 hours and a load of 27,000 p.- s. i. for 100,000 hours. The rods have favorable creep and stress rupture properties at considerably higher temperatures than 1200 F.

Thus it will be seen that in this invention high-phosphorus austenitic stainless articles are provided in which the various objects noted, to-

gether with many thoroughly practical advantages are successfully achieved. It will be seen that the steel products are stron and durable,

corrosion resistant and heat resistant and are well adapted to withstand high temperatures over long periods of service and under varied conditions of actual practical use.

-It will be appreciated that under certain conditions of high temperature use of the articles, reliance-may be had upon the prevailing high temperatures to achieve precipitation for enhancing the load-carrying capacity and resistance to creep.

As many possible embodiments may be made of my invention and as many changes may be made in the embodiment hereinbefore set forth,

, it is tobe understood that all matter described herein is to be interpreted as illustrative and not as a limitation.

I claim:

1. Heat-hardened stainless steel [characterized by resistance to creep and stress rupture, the steel being] of austeniti'c chromium-nickel grade free of amounts of ferrite beyond about 2% by volume, and containing, in addition to about 16% to 26% chromium and 6% to 30% nickel, about 0.05% to 0.5% phosphorus and the remainder substantially all iron, and said steel having a finely divided precipitate in the matrix for slip interference at high temperatures produced by annealing at [2000 F. to 2250 F.] ya. sufiiciently high temperature and for long enough time to form a solid solution of phosphorus and quenching followed by precipitationhardening from 1200 F. to 1500 F.

2. Heat hardened chomium nickel stainless steel high temperature articles characterized by resistance to creep and stress rupture, the steel of said articles being substantially wholly austenitic, containing in approximate percentages.

resistance to creep and stress rupture at high temperatures and containing about 0.10% maximum carbon, 16% to 18% chromium, 10% to 14% nickel, 1.75% to 2.5% molybdenum, 0.05% to 0.5% phosphorus, and the remainder substantialiy all iron, said. steel having afinely divided precipitate in the matrix for slip interference at high temperatures produced by annealing at 511200 FJ! about 2000 F. to 2250 F. and quenching followed by precipitation-hardening at 1200 F; to 1500" F.

t. Age-hardened austenitic chromium-nickel stainless steel articles characterized by freedom from ferrite beyond about 2% by volume and by resistance to creep and stress rupture at high temperatures and containing about 0.10% maximum carbon, 17% to 19% chromium, 8% to 11% nickel, 0.05% to 0.5% phosphorus, titanium at least ten times the carbon content, and the remainder substantially all iron, said steel havin a finely divided precipitate in the matrix for slip interference at high'temperatures produced by annealing at [1200 F.] about 2000" F. to 2250 F. and quenching followed by precipitahon-hardening at 1200 F. to 1500 F.

. 5. Age-hardened austenitic chromium-nickel stainless steel articles characterized by freedom rrom ferrite beyond about 2% by volume and by resistance to creep and stress rupture at high temperatures and containing about 0.10% maximum carbon, 17% to 19% chromium, 9% to 12% nickel,-0.05% to 0.5% phosphorus, columbium at least eight times the carbon content, and the remainder substantially all iron, said steel having a finely divided precipitate in the matrix for slip interference at high temperatures produced by annealing at [1200 FJ about 2000' F. to 2250 F. and quenching followed by precipitation-hardening at 1200 F. to 1500 F.

WILLIAM CHARLES CLARKE, JR.

References Cited in the file of this patent or the original .patenlt UNITED STATES PATENTS Number Name, Date 1,961,777 Palmer Jan. 5, 1934 FOREIGN PATENTS Number Country I Date 375,793 Great Britain June 20, 1932 461,187 Great Britain Feb. 8, 1937 OTHER REFERENCES 

